Conventionally, liquid crystal display devices incorporating nematic liquid crystal display elements have been in widespread use for numeral-segment-type display devices such as watches and calculators, and recently the applications are finding more places with word processors, notebook-type personal computers, liquid crystal televisions mounted in automobiles, etc.
Generally, a liquid crystal display element has a translucent substrate, electrode lines for turning on and off pixels, and other components formed on the substrate. For example, in an active-matrix type liquid crystal display device, active elements, such as thin-film transistors, are formed on the substrate together with the electrode lines as switching means for selectively driving pixel electrodes by which voltages are applied across the liquid crystal. Moreover, in liquid crystal display devices capable of color display, color filter layers having colors such as red, green and blue are provided on the substrate.
Liquid crystal display elements such as the one mentioned above adopt a liquid crystal display mode that is suitably selected depending on the twist angle of the liquid crystal: some of well-known modes are active-driving-type twisted nematic liquid crystal display mode (hereinafter, referred to as the TN mode) and the multiplex-driving-type super-twisted nematic liquid crystal display mode (hereinafter, referred to as the STN mode).
The TN mode displays images by orientating the nematic liquid crystal molecules to a 90.degree. -twisted state so as to direct rays of light along the twisted directions. The STN mode utilizes the fact that the transmittance is allowed to change abruptly in the vicinity of the threshold value of the applied voltage across the liquid crystal by expanding the twist angle of the nematic liquid crystal molecules to not less than 90.degree..
The problem with the STN mode is that the background of the display screen sustains a peculiar color due to interference between colors because of the use of the birefringence effect of liquid crystal. In order to solve this problem and to provide a proper black-and-white display in the STN mode, the application of an optical retardation compensator plate is considered to be effective. Display modes using the optical retardation compensator plate are mainly classified into two modes, that is, the double layered super-twisted nematic optical-retardation compensation mode (hereinafter, referred to as the DSTN mode) and the film-type optical-retardation compensation mode (hereinafter, referred to as the film-addition mode) wherein a film having optical anisotropy is provided.
The DSTN mode uses a two-layered construction that has a display-use liquid crystal cell and a liquid crystal cell which are orientated with a twist angle in a direction opposite to that of the display-use liquid crystal cell. The film-addition mode uses a construction wherein a film having optical anisotropy is disposed. Here, the film-addition mode is considered to be more prospective from the standpoint of light weight and low costs. Since the application of such an optical-retardation compensation mode can improve black-and-white display characteristics, color STN liquid crystal display devices have been achieved that enable color display by installing color-filter layers in STN-mode display devices.
The TN modes are, on the other hand, classified into the Normally Black mode and the Normally White mode. In the Normally Black mode, a pair of polarizer plates are placed with their polarization directions parallel to each other, and black display is provided in a state where no ON voltage is applied across the liquid crystal layer (OFF state). In the Normally White mode, a pair of polarizer plates are placed with their polarization directions orthogonal to each other, and white display is provided in the OFF state. Here, the Normally White mode is considered to be more prospective from the standpoints of display contrast, color reproducibility, viewing angle dependency, etc.
However, in the TN-mode liquid crystal display device, liquid crystal molecules have a refractive index anisotropy .DELTA.n, and are orientated so as to incline to the above and below substrates. For these reasons, the viewing angle dependency increases: i.e., the contrast of displayed images varies depending upon the direction and angle of the viewer.
FIG. 12 schematically shows the cross-sectional construction of a TN liquid crystal display element 31. This state shows liquid crystal molecules 32 slanting upward slightly as a result of application of a voltage for halftone display. In such a liquid crystal display element 31, a linearly polarized ray 35 passing through the surfaces of a pair of substrates 33 and 34 along the normals thereto, and linearly polarized rays 36 and 37 passing through those surfaces not along the normals thereto cross the liquid crystal molecules 32 at different angles. The liquid crystal molecules 32 have a refractive index anisotropy .DELTA.n. Therefore, the linearly polarized rays 35, 36 and 37, upon passing through the liquid crystal molecules 32 in different directions, produce ordinary and extraordinary rays. The linearly polarized rays 35, 36 and 37 are converted to elliptically polarized rays according to the phase difference between the ordinary and extraordinary rays, which cause the viewing angle dependency.
In addition, in an actual liquid crystal layer, the liquid crystal molecules 32 show different tilt angles in the vicinity of the midpoint between the substrates 33 and 34 and in the vicinities of the substrates 33 and 34. The liquid crystal molecules 32 are twisted by 90.degree. around the normal.
For those reasons described so far, the linearly polarized rays 35, 36 and 37 passing through the liquid crystal layer are affected by the birefringence effect in various ways depending upon, for example, the directions and the angles thereof, resulting in complex viewing angle dependency.
Such viewing angle dependency can be observed, as examples, in the following situations. If the viewing angle increases from the normal to the display screen in the standard viewing direction, i.e. downward, and exceeds a certain angle, the displayed image has a distinct color (hereinafter, referred to as the coloration phenomenon), or is reversed in black and white (hereinafter, referred to as the tone reversion phenomenon). If the viewing angle increases from the normal in the opposite viewing direction, i.e. upward, the contrast decreases abruptly.
The aforementioned liquid crystal display device has another problem that the effectual range of viewing angle narrows with a larger display screen. When a large liquid crystal display device is viewed from a short distance in the front thereof, the same color may appear different in the uppermost and lowermost parts of the large screen due to the effect of the viewing angle dependency. This is caused by a wider range of viewing angle required to encompass the whole screen surface, which is equivalent to a viewing direction which is increasingly far off center.
To restrain the viewing angle dependency, Japanese Laid-Open Patent Applications No. 55-600/1980 (Tokukaisho 55-600) and No. 56-97318/1981 (Tokukaisho 56-97318) suggest that an optical retardation compensator plate (retardation compensator film) be inserted as an optical element having optical anisotropy between the liquid crystal display element and one of polarizer plates.
According to the method, the elliptically polarized ray converted from a linearly polarized ray by passing through liquid crystal molecules having refractive index anisotropy is directed through the optical retardation compensator plate(s) disposed on the side(s) of the liquid crystal layer having refractive index anisotropy. Hence, the phase difference between the ordinary and extraordinary rays which occurs to the viewing angle are compensated for, and the elliptically polarized ray is converted back to the linearly polarized ray, which enables the restraint of the viewing angle dependency.
Japanese Laid-Open Patent Application No. 5-313159/1993 (Tokukaihei 5-313159), as an example, discloses an optical retardation compensator plate of the above kind having a refractive index ellipsoid with one of the principal refractive indices parallel to the normal to the surface of the optical retardation compensator plate. Nevertheless, this optical retardation compensator plate still cannot satisfactorily restrain the tone reversion phenomenon that occurs when the viewing angle increases in the standard viewing direction.
Hence, Japanese Laid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116) suggests the use of an optical retardation compensator plate having a refractive index ellipsoid with the principal refractive indices inclining to the normal to the surface of the optical retardation compensator plate. This method adopts two kinds of optical retardation compensator plates as follows.
One of the optical retardation compensator plates has such a refractive index ellipsoid that the smallest of the three principal refractive indices is parallel to the surface, one of the two larger principal refractive indices inclines to the surface of the optical retardation compensator plate by an angle .theta., the remaining principal refractive index inclines to the normal to the optical retardation compensator plate by the same angle .theta., and the angle .theta. satisfies 20.degree..ltoreq..theta..ltoreq.70.degree..
The other optical retardation compensator plate has a refractive index ellipsoid inclining to the surface, where the three principal refractive indices, na, nb, and nc, are mutually related by the inequality na=nc&gt;nb, and the direction of the principal refractive index nb parallel to the normal to the surface and the direction of either the principal refractive index na or nc in the surface recline either clockwise or counterclockwise around the direction of the principal refractive index nc or na in the surface.
As for the former optical retardation compensator plate, a uniaxial and biaxial optical retardation compensator plate can be used. For the latter one, two optical retardation compensator plates, instead of one, can be used in such a combination that the two principal refractive indices nb form an angle of 90.degree..
A liquid crystal display device, incorporating at least one such optical retardation compensator plate having an inclining refractive index ellipsoid between the liquid crystal display element and the polarizer plate, exhibits some restraint in the contrast variations, coloration phenomenon, and tone reversion phenomenon caused by the viewing angle dependency of the display screen, in comparison with incorporation of an optical retardation compensator plate having a refractive index ellipsoid with the principal refractive index not inclining to the normal of the surface.
In order to eliminate the tone reversion phenomenon, Japanese Laid-Open Patent Application No. 57-186735/1982 (Tokukaisho 57-186835) discloses the so-called pixel dividing method, in which a displayed pattern (pixel) is divided and orientation is controlled so that each divided segment has its own viewing angle characteristics independent from those of the other segments. According to the method, since the liquid crystal molecules stand upwards in different directions from segment to segment, the viewing angle dependency when viewed upward or downward can be eliminated.
Japanese Laid-Open Patent Applications No. 6-118406/1994 (Tokukaihei 6-118406) and No. 6-194645/1994 (Tokukaihei 6-194645) disclose technologies to combine the pixel dividing method and an optical retardation compensator plate.
The liquid crystal display device disclosed in Japanese Laid-Open Patent Application No. 6-118406/1994 includes an optical anisotropic film (optical retardation compensator plate) interposed between the liquid crystal panel and the polarizer plate to, for example, improve the contrast. The retardation compensator plate (optical retardation compensator plate) disclosed in Japanese Laid-Open Patent Application No. 6-194645/1994 is set to have almost no refractive index anisotropy in a plane parallel to the surface of the retardation compensator plate and to have a smaller refractive index in a plane perpendicular to the surface of the retardation compensator plate than the refractive index in a plane parallel thereto, in order to have a negative refractive index. Therefore, when a voltage is applied, the positive refractive index occurring to the liquid crystal display element is compensated for and viewing angle dependency can be decreased.
However, with today's increasingly large demand on a wider effectual range of viewing angle and superb display quality, a better restraint in the viewing angle dependency is crucial. In this context, the optical retardation compensator plate having an inclining refractive index ellipsoid as disclosed in Japanese Laid-Open Patent Application No. 6-75116/1994 (Tokukaihei 6-75116) does not provide satisfactory solutions and needs to be improved.
The pixel dividing method, introduced to eliminate the tone reversion phenomenon, does eliminate the tone reversion phenomenon when viewed up or down. However, the introduction of the method suffers from disadvantages that the contrast is lowered and causes the black color to appear rather whitish, resulting in a grey view, and that the viewing angle dependency persists when viewed from the right- or left-hand direction.
The above-mentioned application of the pixel h dividing method to the use of this optical retardation compensator plate results in coloration phenomenon when the viewing angle is increased up to 45.degree. in an oblique direction. Also, since adopting liquid crystal display elements of which the pixels are divided equally (into equal volumes), the application has a limited ability in controlling the decrease of the contrast when viewed up or down. The reasons are elaborated below.
The above-mentioned pixel dividing method adopts pixels divided in a ratio of equality, averaging the viewing angle characteristics of TN liquid crystal display elements in the standard viewing direction (the direction in which the display contrast is improved from the normal to the display surface) and in the opposite viewing direction (the direction in which the display contrast is lowered from the normal to the display surface). However, since the real viewing angle characteristics in the standard viewing direction and those in the opposite viewing direction are reversed, it is difficult to uniformly restrain the decrease in contrast in the upward and downward directions by means of the above-mentioned application of the pixel dividing method to the use of an optical retardation compensator plate. Especially, when the viewing angle increases in the standard viewing direction, it is likely that the tone reversion phenomenon occurs and that displayed images are too dark to be decipherable.